A nickel base alloy comprising, on a weight percent basis, 12-30% chromium, 0.5-3% niobium, 1-5% titanium, 0.2-3% aluminum and the remainder nickel and incidental impurities was, for example, proposed by Japanese Patent Public Disclosure No. 170445/87, an example of said nickel base alloy is comprised essentially of the .gamma. phase matrix and intermetallic compound phases, such as .gamma.' phase Ni.sub.3 (Al,Ti) and .gamma." phase Ni.sub.3 Nb precipitated in the matrix, and exhibits a uniform microscopic structure and orientation and also exhibits excellent tensile strength, corrosion resistance and stress corrosion cracking resistance.
This alloy has been used in producing the above-mentioned bolts and pins as it exhibits excellent corrosion resistance and high tensile strength both characteristics being required for such pins and bolts.
Although this conventional precipitation strengthening type nickel base single crystal alloy exhibits high tensile strength and excellent corrosion resistance when exposed to a corrosive environment under ordinary circumstances, it cannot exhibit satisfactory corrosion resistance when stresses are applied to the alloy articles, such as bolts. Accordingly, the conventional nickel alloy has a tendency to crack due to stress corrosion. On the other hand, since the nickel alloy has a relatively wide solidification temperature range, a pulling down velocity of a melt in the mold in production of a single structure casting structure using a unidirectional solidifying furnace must be made considerably slow. This means that control of a pulling down velocity for producing the single crystal structure in castings, particularly in large sized castings, is very difficult.
With a view to imparting excellent stress corrosion resistance to the conventional precipitation strengthening nickel base alloy while maintaining high tensile strength and excellent corrosion resistance, efforts are being made to develop a novel precipitation strengthening type single crystal nickel base alloy. As a result of such efforts, the inventors of the present invention have found that the stress corrosion crack resistance of the conventional nickel alloy can be improved by adding small amounts of copper thereto and further improved by adding small amounts of hafnium or rhenium or both and that the corrosion resistance of the alloy can be improved by adding small amounts of molybdenum or tungsten or both to the alloy. As a result of studies conducted with the aim of narrowing the solidification temperature range of the conventional precipitation strengthening type nickel base single crystal alloy, the inventors have also found that the solidification temperature range can be remarkably narrowed by adding a small amount of tantalum to the alloy and that therefore large sized castings can be produced with a single crystal structure.